After World War II, the U. S. Atomic Energy Commission (AEC) began mass-producing radionuclides, sending out nearly 64,000 shipments of radioactive materials to scientists and physicians by 1955. Even as the atomic bomb became the currency of the Cold War, radioisotopes represented the government's efforts to harness the power of the atom for peace-advancing medicine, domestic energy, and foreign relations. AEC-produced radioisotopes provided physicians with new tools for diagnosis and therapy and equipped biologists to trace molecular transformations from metabolic pathways to ecosystems. Radioisotopes were especially crucial to the emerging field of nuclear medicine, and were used at the frontiers of research in disciplines such as physiology, endocrinology, and biochemistry. The objective of my project is to reconstruct the history of isotopes as they were used by researchers and physicians, from their importance as molecular tags in basic biomedical research to their applications in clinical settings. My study will pay particular attention to how the changing understanding of health risks associated with radiation exposure (some of which had been recognized since the early days of radium) affected the scientific and medical uses of radioisotopes. The resulting book will tell the story of how radioisotopes, simultaneously beneficial and hazardous, transformed biomedicine. My scholarship on the history of the AEC's radioisotope program will further public understanding of the diverse uses of radioactive materials in medical diagnosis, research, and therapy, countering the view that nuclear weaponry was the only significant consequence of atomic energy development. In addition, my book will provide background to troubling radioisotope experiments during and after World War II involving human subjects and patients, a topic that received public scrutiny in the mid-1990s from President Clinton's Advisory Committee on Human Radiation Experiments. While some of these experiments were secret military activities, others were conducted by civilian scientists and physicians using radioisotopes from the AEC, which also regulated all human applications. My study may shed light on whether and how the AEC's commitment to realize civilian benefits from atomic energy hampered its effectiveness in safeguarding radioisotope users and experimental subjects. Radioactive isotopes continue to be widely used in diagnosis and chemotherapy, and my historical account can help patients as well as physicians to understand the circumstances that led to the development of these important medical tools.